Modern Trusts are designed from the start to last 125 years, so it is natural that whoever creates them is very unlikely to still be alive for the whole duration of the Trust. When you pass the Trust will already know who succeeds you and will then benefit from the assets the Trust is protecting. That person will quickly establish who will take over from them. In exactly the same way the Trustees of the Trust are established ahead of time, so everyone knows who takes over from whom.